1. Field of the Invention
This invention relates to an impedance-matching wave filter, particularly to one having an original filter connected in parallel with a match circuit, which can be modulated to produce impedance matching with the original filter to better the condition of return loss and lower the phenomenon of signal fading and signal distortion.
2. Description of the Prior Art
A conventional signal distributor 1, as shown in FIG. 6, has two or more output ports respectively connected with an input port for receiving signals input through the input port. An amplifier could serve as the signal distributor 1 to receive input signals and then amplify and output them.
Generally, the conventional signal distributor 1 has one of the two output ports connected with a filter 2 for filtering unnecessary signals to attain clear signals. However, after one of the two output ports of the signal distributor 1 is connected with the filter 2, the impedances of the signal distributor 1 and the filter 2 cannot be matched with each other, and hence the other output port of the signal distributor 1 can hardly attain clear signals, always resulting in signal fading and signal distortion.
To detect the filtering effect of the filter 2, a RF network analyzer is employed for detecting the degree of the evenness and the condition of the return loss of the pass band of the output terminal of the filter 2. If the degree of the evenness of the pass band of the output terminal is excellent and the return loss is below −18 db, it shows that the filtering effect of the filter 2 is excellent.
FIG. 7 is a waveform diagram showing the degree of the evenness and the condition of the return loss of the pass band of a conventional high pass filter 2, which are detected by the RF network analyzer. In this waveform diagram, Measure 1 (signal 1) is the output waveform of the filter 2, and Measure 2 (signal 2) is the waveform of the return loss. Viewed from Measure 1, it is obvious that the stop wave band of the signal is positioned below mark 4 (Δ4), that is, the wave-band frequency is below 65 MHz. In comparison with Measure 1, the waveform of the return loss of Measure 2, corresponding to the stop wave band of the signal, is from mark 1 to 4 (Δ1˜Δ4), showing that the return loss is 0.117˜0.794 db, not reaching an extent below −18 db; therefore, the signal received by the other output port of the signal distributor 1 is not in a good condition and results in signal distortion and signal fading. Thus, if the signal distributor 1 is employed for transmission of image signals, the images will be unclear and vague.